1. Field of the Invention
The present invention relates to a key sheet module used in an operational key section of a compact electronic instrument such as a mobile phone or the like, more specifically to a key sheet module capable of applying a complicated circuit to a single-sided printed wiring board on which wiring patterns are formed only on one side of the board.
2. Description of Related Art
In compact electronic instruments such as mobile phones or the like, it has been conventional practice to use a key sheet module including a printed wiring board, electric elements such as a chip-type LED, a resistor, a capacitor and a dome-like contact spring which are mounted on the printed wiring board, and a fixation sheet which fixes the contact spring and covers the electric elements.
While it is necessary that some electric elements such as the LED, the resistor, the capacitor or the like be mounted on the printed wiring board, of the key sheet module, it is also required for the key sheet module to be made smaller and thinner to meet the demands to reduce in size and thickness of instruments in which the key sheet module is installed. Therefore, a two-sided printed wiring board provided with wiring patterns on front and back surfaces, or a multiple-layer board making use of insulating layers to form wiring patterns in a multilayered state has been adopted to deal with a complicated circuit structure.
However, because the two-sided printed wiring board or multiple-layer board is expensive, a way of dealing with a complicated circuit structure has been proposed in which jumpers configured to connect remote wiring patterns skipping adjacent wiring patterns are mounted on an inexpensive single-sided printed wiring board (for reference, see Japanese Patent Laid-Open No. 2005-71808).
FIG. 10 illustrates a conventional key sheet module having a structure in which wiring patterns disposed on a single-sided printed wiring board are connected through jumpers. As shown in FIG. 10, a plurality of wiring patterns 4a, 4b, and 4c are provided only on an upper surface of the single-sided printed wiring board 2 at predetermined intervals.
To electrically connect the wiring patterns 4a and 4c by a jumper 8 which comprises a conductive paste and is formed by printing, an insulating layer 6 is disposed on the wiring pattern 4b to be insulated between the wiring patterns 4a and 4c, and the jumper 8 to connect the wiring patterns 4a and 4c is disposed to straddle the wiring pattern 4b and the insulating layer 6 disposed on the wiring pattern 4b. 
An insulating coat 10 is formed on the jumper 8 by printing. The provision of the jumper 8 makes it possible to deal with a more complicated circuit structure even in the single-sided printed wiring board 2.
However, in recent years, even more complicated circuit structures have become necessary, because it is difficult to secure space to provide a number of jumpers on the single-sided printed wiring board.